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Precision Measurement Using Machine Vision (Part 3)

Precision Measurement – Final Thoughts
This is the third and last entry, for now, in my discussion of precision measurement. I’m hoping this series helps eliminate that question that always seems to come up: “How accurately can Camera X measure something?” It should be clear now that the camera or camera brand is not the key gating technology in a measurement application, and that one cannot know the potential precision of a measurement until the target field of view is defined and the resolution capability is known. And to review, we noted that “accuracy” is a term often misused when we really mean “precision” and “repeatability”.

In conclusion, consider the following thoughts about lighting and part features and presentation.

Choose The Correct Illumination

In metrology, the choice of illumination may plays a critical role. Unfortunately there is no specific rule that can be applied to illumination. Many metrology applications benefit from backlighting (taking care in part presentation as noted below), though the physical implementation of a backlight in automation on the production line may be a challenge. Front lighting may present difficulties in highlighting feature edges that must be identified for measurement. Consider the use of low-angle or structured lighting to bring out low contrast features. When attempting to measure features that are very small (with resolutions below 0.001mm for example) use long wavelength colors such as blue or violet to enhance contrast. If the part is in motion, (or even if not), consider strobing the LED illuminator for the best intensity and lamp life.

In all cases, successful machine vision illumination requires experimentation, both in the lab and on the floor, in order to ensure the correct component selection.

Pay Attention to Part Features

The feature that is visible to the camera’s optical system given a particular lighting technique often does not present the same features for machine vision tools as may be specified on a part print or measured using manual gauges. Take, for example, measurement of the diameter – to a high precision and low uncertainty – of a through bore hole that is small in diameter but quite deep. If a front lighting is used, only the top edge of the hole will be gauged. This could be unacceptable if the desired inspection is to mimic an insertion gauge. On the other hand, if back lighting is used, due to the depth of the bore it is unlikely that the optics will “average” the entire bore in the image. More probably, the optics will be focused on a point at some depth in the bore – top, bottom, middle – and again this result might not be the one desired. Choose lighting, optics and algorithms carefully to ensure measurement of an agreed-upon surface. Understand that in many cases, on-line, non-contact machine vision measurement will not exactly duplicate a physical measuring device for the reasons described above.

Part Presentation Can Sabatoge Precision Measurement

Parts under inspection for measurement must be presented repeatably. The imaging, optics, resolution, and algorithms might all be perfect in an off-line setup, but you find that repeatability and reliability of the measurement online is poor. Usually, it’s inconsistency in part presentation. Sometimes part presentation can even make a certain measurement impossible to achieve. Take, for example, the small but deep bore hole described earlier. When the face of that hole is perpendicular to the lens, and the image is taken directly down the depth of the hole, it can be successfully measured. However, if the part tilts even slightly, a hole like that could visibly turn into an ellipse, or be obscured completely if backlit. With imaging for non-contact measurement one must first mitigate all possible variation in part presentation, then understand that in any case part presentation will be responsible for some stack-up error in the measurement. Take that into consideration when determining and specifying resolution, optics, and lighting.